A variety of private branch exchanges (PBX) have been recently commercialized such as a PBX having a computer telephony integration (CTI) function, an ISDN-compliant PBX, and a wireless PBX.
In the foregoing circumstances, a PBX equipped with a plurality of central processing units (CPU) is introduced in the market for dealing with loads applied one after another. In this PBX, a main CPU (main control unit) controls mainly applications in upper layers or carries out an overall control. On the other hand a sub-CPU (a sub-control unit) is mounted to various line-cards and carries out mainly processing in the terminal that needs a real-time process. The main CPU and the sub-CPU thus play their own roles independently, so that the loads are distributed.
FIG. 4 shows a block diagram illustrating a conventional PBX, which comprises main control card 130, and a plurality of line cards 132a, 132b, . . . , 132n. Main control unit 101 of main control card 130 works as a CPU of the PBX. Main memory 102 stores the programs and data of main control unit 101. System-bus master controller 103 converts the data into a format in accordance with its own system-bus standard, converts an address, and adjusts a timing when main control unit 101 accesses respective line cards 132a-132n. System-bus 115 couples the main control card to respective line cards. Highway-bus 116 couples the respective line cards to each other.
In line-card 132a, respective sections work as follows: System-bus slave controller 104 receives an interface signal supplied from system-bus master controller 103 of main control card 130, and determines whether or not the signal accesses card 132a. Only when controller 104 finds the access to card 132a, controller 104 interprets an access mode, and converts the data received into a format of the bus standard in the line card, converts an address, and adjusts a timing. Sub-control unit 105 works as a CPU of one of line cards 132a-132n. Individual memory 106 stores the program and data of sub-control unit 105.
First module group A 107 is directly coupled to local-bus 117, which is a bus of sub-control unit 105, and carries out a process such as data conversion as a coder-decoder (codec). Module group B 108a and module group B 108b produce interface signals to be supplied to first module group A 107. Further the two groups carry out various processes such as a switching control or a gain control over audio data, which is divided and placed in time slots and passes through highway bus 116 in the PBX, and a control over communications in a conference.
Internal and external lines interface 109 is an interface connector between the line card and a group of telephone terminals. Intelligent block 118 comprises local-bus interface 110, direct memory access controller 111 and bi-directional memory 113.
Local-bus interface 110 harmonizes sub-control unit 105 with intelligent block 118 so that smooth access between unit 105 and block 118 can be executed. Direct memory access controller 111 transfers data at a high speed between individual memory 106 and second module groups 108a, 108b without any help from sub-control unit 105. Bi-directional memory 113 temporarily stores a command when the command is transferred between main control unit 101 and sub-control unit 105
In the PBX thus structured, second module group B 108a, which main control unit 101 accesses, is clearly separated in advance from second module group B 108b which sub-control unit 105 accesses, though both of the groups are disposed on the same line card. Since the loads to be applied to the main control unit and the sub-control unit can be predicted according to a system size of the PBX, namely, the number of lines to be supported, the separation and distribution of the loads are determined.
Assume that main control unit 101 works on second module group B 108b that is under the control of sub-control unit 105, in this case main control unit 101 sends a command to sub-control unit 105 via bi-directional memory 113 so that sub-control unit 105 can substitute for main control unit 101.
As discussed above, in the conventional PBX, second module group B 108b can be directly controlled only by sub-control unit 105. Therefore, when sub-control unit 105 falls into an inoperable condition, an operation of the line card having this sub-control unit 105 cannot be guaranteed. In this situation, extensions governed by this line card are in trouble.
Even if first module group A 107, individual memory 106, second module groups B 108a, 108b were accessible directly from both of main control unit 101 and sub-control unit 105, and an operation of the line card were guaranteed, plural stages of bus-arbitration circuits could be needed. Whenever main control unit 101 or sub-control unit 105 accesses a module in a lower layer, and every bus arbitration is carried out, then a speed of access becomes slower, which causes the PBX to work inefficiently.